If they look like baby crawfish, I bet they're amphipods. Google a pic. But if you say it they look like horseshow crabs, it sounds like Argulus (fish lice), and are very nasty critters in the tank.
did u put any driftwood in the tank? one time i put a piece of drift wood in a temp holding tank for a turtle and eventually i noticed this small clear leech looking thing. it was nasty. im never putting another piece of wood in a tank b4 boiling it first